Quantum key distribution involves establishing a key between a sender (“ALICE”) and a receiver (“BOB”) by using weak (e.g., 0.1 photon on average) optical signals (“quantum signals”) transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the quantum signal will introduce errors into the transmitted signals, thereby revealing her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984)(hereinafter, “Bennett & Brassard”). Specific QKD systems are described in publications by C. H. Bennett et al entitled “Experimental Quantum Cryptography,” J. Cryptology 5: 3-28 (1992), and by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992), and in U.S. Pat. No. 5,307,410 to Bennett (the '410 patent). The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33.
U.S. Pat. No. 6,438,234 to Gisin (the '234 patent), which patent is incorporated herein by reference, discloses a so-called “two-way” QKD system, wherein the quantum signals are autocompensated for polarization and thermal variations. In a typical two-way QKD system, a synchronization channel is also run in both directions to coordinate the timing at both QKD stations.
Even the quantum signals are compensated in a two-way system, synchronization (“sync”) signals that travel only one way through an optical system would not be autocompensated for variations in the optical path connecting ALICE and BOB.
Variations in the timing circuits also affect the timing relationship between the arriving quantum signal (which has one photon or less, on average) and the single-photon detector (SPD) detection window. Accordingly, both thermal and electronic timing variations can act to reduce the probability of successfully detecting the quantum signal. While some degree of variation could be compensated by using a wider detection window, this results in an increase in the false positive detections (counts) in the SPD, which reduces system performance.
Other options are available for synchronizing the operation of a two-way QKD system. One option involves sending synchronization signals back and forth between the two QKD stations in the system. However, this is not desirable in the case where the optical fiber link connecting the two QKD stations carries both the quantum signal and the synchronization signal. This is because scattered light from the stronger synchronization signals can interfere with the SPD detection process, particularly when the quantum and synchronization channels operate at the same wavelength.